The present invention relates generally to acidic polymer-based PSAs that are thermosettable to provide semi-structural or structural adhesives.
When bonding substrates for applications that require particularly high bond strength, it may be desirable to use a PSA (PSA) that can be cured to form a high strength bond, or semi-structural or structural bond. Typically, such PSAs are referred to as thermosettable PSAs. An advantage of thermosettable PSAs is that they can be used to bond surfaces together without the need to clamp, or otherwise hold, the substrates together until the PSA is cured.
For example, see U.S. Pat. No. 5,593,759 (Vargas et al.), where a tape constructed of a core layer of structural adhesive is coated with PSA layers (e.g., acrylic PSAs). The multi-layer bonding tape construction can then be adhesively applied between parts to be bonded. The tape is then cured to form a structural bond. The core layer can be made of various different structural adhesives, such as a partially cured, B-stage structural adhesives or blends of an epoxy-containing material with an acrylate ester resin and hardener. Examples of hardeners include: trichloride amine complexes, boron trifluoride complexes, monoethyl amine, blocked amines, or dicyandiamide and the like. The hardeners are preferably incompatible with the resin at room temperature. It is not always desirable, however, to use a multi-layer adhesive when bonding. Multi-layer adhesives, such as those comprising a core layer coated with PSA layers, often require more complicated preparation procedures as compared to, for example, a single layer bonding adhesive.
Other adhesive systems relying on a combination of epoxy-containing materials and acrylate resins are also known. Similarly, these systems employ a curative, such as an amine-containing compound, which is typically referred to as an epoxy curative. See, for example, PCT Publication Number WO 96/21,704 (Minnesota Mining and Manufacturing Co.) and U.S. Pat. Nos. 4,612,209 (Forgo et al.); 5,011,560 (Nakai et al.); 5,086,088 (Kitano et al.); 5,686,509 (Nakayama et al.); and 5,883,193 (Karim).
It may not always be desirable to use bonding adhesives that include epoxy-containing materials, however. Epoxy-based adhesive systems (i.e., those systems containing epoxy-containing materials as the major component of the adhesive) generally rapidly lose their bond strength at temperatures above their glass transition temperatures as compared to, for example, (meth)acrylate-based adhesive systems (i.e., those systems containing (meth)acrylates as the major component of the adhesive). Furthermore, known adhesive systems comprising epoxy-containing materials are oftentimes brittle and, thus, may prove to be particularly difficult, for example, in effectively bonding irregular (e.g., nonplanar) surfaces or in being used in those applications where a certain degree of flexibility is required, or desired, in the resulting bond. For example, adhesives used in vibration damping articles may benefit from a certain degree of flexibility so as to allow the adhesive to absorb vibrations incident on the article. Other known epoxy-based adhesive systems may also lack moisture resistance, impact strength, or storage stability desired, or required, for a particular application.
As alternatives to epoxy-based adhesive systems, certain references describe crosslinking of (meth)acrylate polymers using amine-containing compounds similar to those described for use in epoxy-based adhesive systems. For example, U.S. Pat. No. 2,925,174 (Stow) describes crosslinking acrylate PSAs using certain polyfunctional polymeric amines. However, the crosslinked compositions are PSAs, which do not have enough bond strength for applications requiring semi-structural or structural bond strengths.
U.S. Pat. No. 5,596,039 (Oishi et al.) describes the use of certain diguanamines prepared from dicyandiamides for thermosetting compositions, including acrylic resins. The resins are purportedly useful in adhesive applications.
U.S. Pat. No. 4,404,246 (Charbonneau et al.) describes a PSA tape having an adhesive layer of a copolymer of alkyl acrylate and an acid, and a small amount of a lower-alkoxylated amino formaldehyde condensate as a latent crosslinking agent. After the tape has been applied and heated to crosslink the adhesive, the resultant bonds have outstanding resistance to peel forces combined with good resistance to shear forces, especially at elevated temperatures.
However, the lower-alkoxylated amino formaldehyde described in this reference contains tertiary amines and the lower-alkoxylated amino formaldehyde is used in amounts within the range of about 0.1-0.8% by weight of the copolymer. Furthermore, the adhesive must be heated moderately, such as at 120xc2x0 C. for 30 minutes, to crosslink (i.e., cure) the adhesive. The more heat that is required during a process, however, the more energy must be expended, resulting in processes that take longer and cost more.
Similarly, see U.S. Pat. No. 4,396,675 (Groff). Again, the alkoxylated crosslinking agent contains tertiary amines. The crosslinking agent is used in amounts of about 0.1 to about 10 percent by weight of the copolymer, preferably 1 to 5 percent. Groff states that amounts above 10 percent would not enhance crosslinking and might detract from the adhesive properties of the tape.
Depending on the application and type of substrate being bonded, the use of moderate to high temperatures for curing the adhesive may also be problematic. This may be the case, for example, when heat-sensitive substrates are to be bonded. Heat-sensitive substrates include relatively low softening point or melting temperature plastics, such as polycarbonate, polyethylene terephthalate polyester, polymethyl methacrylate, and certain polyolefins, as well as substrates, such as electronic circuit boards, having heat-sensitive components attached thereto.
Other compositions comprising (meth)acrylates and amine-containing compounds are also known. For example, aqueous coating compositions comprising dicyandiamide dissolved in an aqueous solution of a salt of a copolymer of an unsaturated carboxylic acid monomer and a hydroxylalkyl (meth)acrylate monomer are disclosed in U.S. Pat. No. 4,045,399 (Suzuki et al.). Also see German Patent Publication Numbers DE 3634780 and DE 3726956 (both to Kruger). Furthermore, see Czechoslovakian Patent Number 227 514. However, when drying the aqueous coating compositions to remove water from the system, the coating compositions may become cured. Therefore, intermediate thermosettable adhesives may not be possible when using such aqueous systems and coating methods described therein. Furthermore, types and amounts of components used in aqueous systems must be carefully selected so as to be soluble in water, limiting formulation latitude.
Vulcanizable acrylic rubber compounds comprising dicyandiamide are described in Japanese Patent Publication Numbers JP 61-076542; JP 61-278554; and JP 61-1103957. Powder coating compositions comprising acrylic polymers and dicyandiamide are described in Japanese Patent Publication Number 51-057723.
Alternative thermosettable PSAs are desired. Such compositions would be particularly useful for bonding substrates without the need to clamp, or otherwise hold, the substrates together until the PSA is cured. It would also be desirable for the PSAs to be capable of being cured to form high bond strength adhesives, such as semi-structural or structural bond adhesives.
Thermosettable PSA (PSA) compositions of the invention comprise a major proportion of the adhesive component of at least one acidic polymer and at least one amine-containing compound capable of reacting with acidic functional groups on the acidic polymer to cure the thermosettable PSA composition into a thermoset adhesive. The thermosettable PSA compositions are particularly useful for forming semi-structural or structural bonds.
In one embodiment, the thermosettable PSA composition comprises 100 parts by weight of an adhesive component comprising at least about 50 percent by weight of the adhesive component of at least one acidic polymer and, optionally, no more than about 10 percent by weight of the adhesive component of at least one epoxy-containing material; and about 1 to less than about 50 parts by weight of at least one amine-containing compound capable of reacting with acidic functional groups on the acidic polymer to cure the thermosettable PSA composition into a thermoset adhesive.
In another embodiment, the thermosettable PSA composition comprises 100 parts by weight of an adhesive component comprising at least about 50 percent by weight of the adhesive component of at least one acidic polymer and, optionally, no more than about 25 percent by weight of the adhesive component of at least one epoxy-containing material, wherein the acidic polymer is derived from at least about 2% by monomer weight of at least one acidic monomer; and about 1 to less than about 50 parts by weight of at least one amine-containing compound capable of reacting with acidic functional groups on the acidic polymer to cure the thermosettable PSA composition into a thermoset adhesive.
For optimum curing capabilities, preferably the amine-containing compound is capable of curing the thermosettable PSA composition at temperatures of less than about 150xc2x0 C., more preferably at temperatures of less than about 120xc2x0 C., and even more preferably at temperatures of less than about 100xc2x0 C. It is also preferred that the amine-containing compound is a solid at both room temperature and temperature at which the amine-containing compound is capable of curing the thermosettable PSA composition. Similarly, it is also preferred that the amine-containing compound is essentially insoluble or immiscible with the acidic polymer at room temperature and temperatures up to temperatures at which the amine-containing compound is capable of curing the thermosettable PSA composition.
In certain embodiments, the amine-containing compound comprises about 5 to about 20 parts by weight of the thermosettable PSA composition. However, it is to be understood that the thermosettable PSA composition can contain more or less of the amine-containing compound, depending on the particular application.
Preferably, the amine-containing compound comprises at least two amines selected from primary amines, secondary amines, and mixtures thereof. Particularly useful are non-aromatic amine-containing compound and those amine-containing compounds having a molecular weight of about 500 grams/mole or less. One example of a particularly useful amine-containing compound is dicyandiamide.
A wide variety of acidic polymers may be used. Preferably, the acidic polymer comprises a copolymer derived from at least one carboxylic acid, such as (meth)acrylic acid (e.g., acrylic acid). Preferably, the acidic polymer comprises an acidic (meth)acrylate polymer, such as a polymer derived from isooctyl acrylate and acrylic acid.
In certain embodiments, the acidic polymer comprises a polymer derived from up to about 25 percent by monomer weight of at least one acidic monomer. In further embodiments, the acidic polymer comprises a polymer derived from about 2 to about 15 percent by monomer weight of at least one acidic monomer.
Due to the disadvantages associated with conventional epoxy-based adhesives, preferably, the adhesive component comprises no more than 0 to about 5 percent by weight of at least one epoxy-containing material. Most preferably, the adhesive component is essentially free of epoxy-containing materials.
In one embodiment, thermoset adhesives of the invention are capable of being used as semi-structural adhesives. In yet another embodiment, thermoset adhesives of the invention are capable of being used as structural adhesives. The thermoset adhesive compositions are preparable by curing thermosettable PSA compositions of the invention.
For example, in one embodiment, a thermoset adhesive of the invention comprises the reaction product of: 100 parts by weight of an adhesive component comprising at least about 50 percent by weight of the adhesive component of at least one acidic polymer and, optionally, no more than about 10 percent by weight of the adhesive component of at least one epoxy-containing material; and about 1 to less than about 50 parts by weight of at least one amine-containing compound capable of reacting with acidic functional groups on the acidic polymer to cure the thermosettable PSA composition into the thermoset adhesive.
In another embodiment, a thermoset adhesive of the invention comprises the reaction product of: 100 parts by weight of an adhesive component comprising at least about 50 percent by weight of the adhesive component of at least one acidic polymer and, optionally, no more than about 25 percent by weight of the adhesive component of at least one epoxy-containing material, wherein the acidic polymer is derived from at least about 2% by monomer weight of at least one acidic monomer; and about 1 to less than about 50 parts by weight of at least one amine-containing compound capable of reacting with acidic functional groups on the acidic polymer to cure the thermosettable PSA composition into a thermoset adhesive.
Thermosettable PSA compositions of the invention can be coated on at least a portion of any suitable substrate. For example, composites comprising a first substrate, a second substrate, and a thermosettable PSA composition positioned therebetween can be prepared. Once cured, the substrates are at least partially coated with a thermoset adhesive composition of the invention. In the case of composites, once cured, the composites comprise a thermoset adhesive composition positioned between the first and second substrates.
Articles comprising thermosettable PSA compositions and thermoset adhesive compositions of the invention include, for example, tapes. In one embodiment, a tape of the invention comprises a backing, and a layer of the thermosettable PSA composition on at least a portion of at least one side of the backing. In another embodiment, a transfer tape of the invention comprises a release liner, and a layer of the thermosettable PSA composition on at least a portion of the release liner.
Methods for bonding a substrate comprise the steps of: providing the substrate to be bonded, adhering a layer of the thermosettable PSA composition to at least one side of the substrate, adhering the thermosettable PSA layer to a surface to be bonded, and curing the thermosettable PSA to form a thermoset adhesive between the substrate and the surface. Preferably, the step of curing the thermosettable PSA comprises heating the thermosettable PSA at a temperature of less than about 150xc2x0 C., more preferably at a temperature of less than about 120xc2x0 C., and even more preferably at a temperature of less than about 100xc2x0 C.